Digital communication systems provide exchange of data between devices, wherein data bits are typically transferred over a communication medium in groups (packets). For example, in telecommunications or other networked systems, packets of data are transmitted from a source device to a destination device, where the network communication protocol may call for segmentation of a data packet into a number of chunks or frames that are separately transferred to provide services such as internet browsing, voice over IP, digital video, Ethernet, etc. Networks are typically implemented as a series of layers, and described as a multi-layered model, for example, the Open Systems Interconnection (OSI) model, in which a first (lowest) layer is the physical layer at which electrical signals are actually transferred, and a second OSI layer is known as the data link layer that is between the physical layer and a network layer. The main function of the data link layer (layer 2) is to make the physical layer appear to the network layer (layer 3) as a transmission line that is free of transmission errors. In the data link layer, the input data packet is encapsulated into data frames that are then provided to the physical layer in sequence for transmission to the destination device, where the data link layer may also process acknowledgment frames sent back by the receiver.
In most systems, a unit of data being transferred to a destination device is provided from an upper interconnection layer, such as the network layer, to the data link layer, which then forms one or more frames that encapsulate the data packet according to an encapsulation or framing protocol or procedure. The physical layer accepts and transmits a stream of bits regardless of meaning or structure, whereas the data link layer creates or defines frame boundaries, sometimes by using special bit patterns appended to the beginning and/or end of the transmitted frame. In this context, an encapsulation or framing protocol is a group of rules for transferring a data packet across a communication medium to a destination device that verifies whether the data has been successfully received, wherein the protocol is needed to ensure that the destination device can recognize the boundaries of the transmitted packet to identify and decode the contents thereof. In general, it is desirable to maximize data throughput by minimizing the amount of overhead or identification (alignment) bytes that are added to the packet in creating a frame for transmission. Currently, there are many protocols used for transporting packets, including the International Telecommunication Union (ITU-T) standardized General Framing Procedure (GFP), which allows transfer of any type of packet over any media. The GFP protocol, however, suffers from relatively high overhead (e.g., at least six overhead bytes per packet), as well as rather complex synchronization.
Another popular protocol is known as High-level Data Link Control (HDLC, ISO/IEC 3309), which was standardized by the International Telecommunication Union (ITU-T) for packetized data transport in Very High Digital Subscriber Line (VDSL) and Asynchronous Digital Subscriber Line (ADSL) communications. Digital subscriber line (DSL) technology provides high-speed data transfer between two modems across ordinary telephone lines, wherein digital data transfer rates from tens of Kbps to tens of Mbps are supported over standard (e.g., twisted pair) telephone lines, while still providing for plain old telephone service (POTS). ADSL and VDSL have emerged as popular implementations of DSL systems, where ADSL is defined by American National Standard Institute (ANSI) standard T1.413 and ITU-T standards G.992.3, G.992.5, and VDSL is defined by ANSI standard T1.424 and ITU-T standards G.993.1. HDLC is a data link layer encapsulation protocol that allows transport of any type of data packet. However, due to the specific byte-stuffing mechanism introduced to avoid false detection of HDLC frames, the amount of overhead introduced for transmission using HDLC depends on the packet data contents. Because the overhead is variable, however, HDLC was found inappropriate for Ethernet data transport.
More recently, the Institute of Electrical and Electronics Engineers (IEEE) has proposed a new protocol for Ethernet transport over DSL, using 64/65-byte encapsulation, also known as 64B/65B, which has low, stable overhead (1 overhead sync byte per 65 byte frame), where the overhead is largely independent of the packet size and contents. However, the use of the 64B/65B protocol imposes limitations on the size of packets to be transmitted, and thus inhibits adoption of this protocol in situations where a universal protocol is desired for multiple types of services beyond Ethernet. Moreover, the 64B/65B protocol currently does not facilitate efficient transport of packets of different priorities or Quality Of Service (QOS) levels. It is desirable to provide a universal framing protocol for DSL and other communication systems, which supports Ethernet as well as other protocols, such as IP, for instance, and which allows encapsulation or framing of packets of different priorities (usually associated with QOS levels) in an efficient manner.
A capability to prioritize some packets of the transmitted packet stream relative to others is necessary when this packet stream is shared by services with different latency requirements and delay variation requirements. As high-priority packets could be send without waiting until the previously sent low-priority packets are transmitted, various delay-sensitive services, such as voice or video teleconferencing, could be delivered with high quality even through a relatively low-speed access network. Accordingly, there is a need for improved communication data packet encapsulation methods and protocols for use in transferring data in a DSL or other communication system. Also, since Ethernet is currently one of the most widely used protocols, it is desirable that new encapsulation methods will be based on the same principal as the standard IEEE 64B/65B encapsulation to simplify implementations intended to operate universally.